How to Choose a Planetary Gearbox: Torque, Ratio, IP Rating & Mounting

What Is a Planetary Gearbox — and Why Is It Used?

A planetary gearbox — also called a planetary reducer or epicyclic gearbox — uses a central sun gear, a set of planet gears that orbit it, and a fixed outer ring gear. This arrangement distributes load across multiple gear contact points simultaneously, which gives planetary designs their defining characteristic: high torque density in a compact housing.

Compared to a conventional parallel-shaft gearbox of the same torque rating, a planetary unit is typically 30–50% smaller and lighter. The inline (coaxial) input and output layout also makes integration into machinery cleaner than helical or worm designs.

Planetary gearboxes are used across a wide range of industries: track drives and winch drives in construction equipment, slewing drives in solar trackers and cranes, feed mixer drives in agricultural TMR wagons, wheel drives in AGVs and forklifts, and precision servo gearboxes in CNC machines and robotic systems. Each application has different specification priorities — which is why a clear selection process matters.

Step 1: Define Your Required Output Torque (Nm)

Output torque is the primary sizing parameter. Get this wrong in either direction and everything else follows incorrectly. The formula is straightforward:

For example: a motor producing 50 Nm through a planetary gearbox with a ratio of 50:1 and efficiency of 0.97 delivers approximately 2,425 Nm at the output shaft.

In practice, you also need to account for peak torque, not just continuous rated torque. Most industrial applications experience load spikes — starting loads, impact loads, shock loads — that exceed the continuous operating torque by a factor of 1.5 to 3×. Feed mixer wagons, for example, are typically rated for 3× continuous torque at start-up due to the inertia of the loaded mixing drum.

Always specify the gearbox with a service factor applied — typically 1.25 to 2.0 depending on application severity. Your gearbox supplier should be able to confirm the service factor recommendation for your specific duty cycle.

Step 2: Determine the Required Gear Ratio

The gear ratio determines how much the input shaft speed is reduced at the output. A 10:1 ratio means 1,000 RPM input becomes 100 RPM output (with torque multiplied by 10, minus efficiency losses).

Standard single-stage planetary gearboxes cover ratios from approximately 3:1 to 10:1. Multi-stage units extend this to 10,000:1 for very slow output speeds. The table below gives typical ratio ranges by number of stages:

Note that adding stages increases backlash and reduces transmission efficiency slightly. If your application requires very high positional accuracy (servo systems, CNC machines), specify a high-precision low-backlash planetary gearbox rated at ≤3 arcmin — rather than stacking stages on a standard unit.

Step 3: Select the Right IP Rating for Your Environment

IP rating (Ingress Protection) defines how well the gearbox housing resists dust and water. The two-digit IEC 60529 rating covers solid particle protection (first digit) and liquid protection (second digit). Choosing the correct IP rating for your operating environment is as important as selecting the correct torque rating.

⚠️ Underspec’ing IP rating is one of the most common causes of early seal failure. If your equipment operates outdoors in variable conditions, specify one level above what you think you need. Upgrading from IP65 to IP67 typically adds less than 5% to the unit cost — seal replacement labour and machine downtime costs far more.

Step 4: Choose the Mounting Configuration

Planetary gearboxes are available in several mounting configurations. The right choice depends on how the gearbox integrates into your machine’s drivetrain and structural envelope:

• Foot mount (torque arm): The gearbox body is supported by a foot bracket, with the output shaft connected to the load. Used in inline conveyor and mixer drives where the motor and gearbox are mounted on a base frame.
• Flange mount (IEC B5/B14): The gearbox is bolted directly to the machine structure via a pilot flange. The most common configuration for industrial machinery. Standardised flange dimensions allow direct motor mounting without coupling.
• Hollow shaft output: The output shaft is hollow, allowing the gearbox to slip directly onto a keyed input shaft of the driven equipment. Eliminates the need for a coupling and reduces overall drive length. Common in wheel drives, feed mixers, and track drives.
• Hub mount: The entire gearbox integrates into the hub of a wheel or sprocket. No external shaft — the output flange is the wheel hub mounting face. Used in all wheel drive and track drive applications.
• Right angle output: A bevel or worm gear stage is added to deliver output at 90° to the input. Used when inline shaft arrangement is physically impossible. Slightly lower efficiency than inline configurations.

For OEM replacement applications, the mounting configuration is already determined by the machine design. Specify the same configuration as the original unit and verify that the gearbox supplier can confirm dimensional equivalence to the OEM part.

Step 5: Specify Motor Input Interface

The input interface must match the motor that drives the gearbox. There are two main types:

• Electric motor adaptor (IEC standard flange): A standardised adaptor ring allows any IEC-frame electric motor to bolt directly onto the gearbox input. This is the standard for industrial conveyors, mixers, and positioning drives. The adaptor size must match the motor’s IEC frame designation (e.g. IEC 100, IEC 132).
• Hydraulic motor SAE flange: For mobile equipment driven by hydraulic motors, the input uses SAE standard bolt patterns (SAE A, B, C, or D). Wheel drives, track drives, and winch drives almost universally use hydraulic motor inputs. Verify the SAE flange size and motor shaft diameter.

For servo motor applications — CNC machines, robots, packaging lines — a high-precision servo-compatible input is required. These gearboxes have a precisely dimensioned input flange that accommodates direct servo motor mounting without a separate adaptor ring. Ensure the servo flange pattern matches your motor brand (Siemens, Fanuc, Yaskawa, etc.) before ordering.

Quick Reference: Planetary Gearbox Selection by Application

OEM Replacement vs New Specification: What Information to Provide Your Supplier

How you communicate your requirement to a supplier determines whether you receive the right unit on the first order. The information needed differs depending on whether you are replacing an existing gearbox or specifying a new one:

A reputable supplier will return a formal quotation within 24 hours for standard replacement parts, and a technical proposal with product recommendation within 48 hours for new specifications. If your application involves significant shock loading, variable duty cycles, or extreme temperatures, expect your supplier to ask follow-up questions — this is a good sign that they are properly engineering the selection rather than simply picking the nearest catalogue size.